The first thing to consider is that we're not talking about a bunch of Italians ruling in isolation over a subjugated and tattooed population of Celts, seething to overthrow their Latin masters. Britannia was a well-established province, and there was a good deal of co-mingling. There was relatively organized life in the towns, and rule of law, with the magistrates and other officals to direct and enforce it. When Imperial control was finally withdrawn, it didn't mean a complete break with Roman law and custom, but it did mean that the province was now open more than ever to incursions from the unconquered areas of the island, and that there was nothing to stop a local nobleman from taking contral and establishing patronage. Into this disorganized mix, some locals invited Angles, Jutes and Saxons as mercenaries to replace the legions. And as happened elsewhere in the Roman empire, once they arrived, the barbarians decided they liked the country and they invited more of their tribesmen to come and settle, and they established themselves as armed chiefs. There was then a mixing of the Germanic tribal life with the Roman British culture. Eventually they established themselves in their small kingdoms, and fell prey themselves to the later Danish and Norse incursions.
Simon Schama's "A History of Britain" provides an excellent overview of, well, the History of Britain, including the period after the decline of Roman rule. That's where I get the outline above.
Prost!
Brad